For precision cooling of a liquid for temperature sensitive operations, it would be highly desirable to achieve a rapid system start up from low ambient temperature, to keep the chilled liquid temperature nearly constant over varying load conditions, and to be able to control the discharge of system heat. Because ambient temperatures may be low am start up, and may vary during the operation of the liquid chiller, an effective liquid chiller must be able to compensate. To attain these desirable characteristics, an effective liquid chiller must respond to changing heat/temperature/pressure conditions within the system, as those conditions may be influenced by the ambient conditions surrounding the system.
Refrigeration systems for chilling a liquid are generally old and well-known in the art. Such systems of the vapor compression type conventionally include a compressor, a condenser, an expansion device, and an evaporator, with a refrigerant circulating repeatedly through the system.
In the evaporator, the refrigerant boils (evaporates) at a temperature sufficiently low to absorb heat from a space or from a medium that is being cooled. The evaporating temperature is determined, for any given refrigerant, by the pressure maintained in the evaporator--the higher the pressure, the higher the boiling point; the lower the pressure, the lower the boiling point.
The compressor pulls and removes vapor from the evaporator as the vapor is formed, at a rate sufficiently rapid to maintain the desired pressure in the evaporator. The vapor is then compressed and delivered to the condenser.
The condenser dissipates heat contained in the hot vaporized refrigerant to a circulating coolant, usually ambient air, although water or brine could be used as well. The refrigerant is condensed to a liquid and is ready for circulation, through an expansion device, back to the evaporator.
Between the condenser and the evaporator is a flow restriction device or an expansion device such as a valve. The expansion device sharply reduces the pressure of the liquid refrigerant passing through it, thereby reducing the pressure and temperature of the refrigerant until they reach the evaporator pressure and temperature, or, put another way, until they reach the level maintained by the suction line of the compressor as it pulls vapor from the evaporator.
The vapor compression and expansion refrigeration process just described depends upon a refrigerant which absorbs heat at a relatively low temperature (in the evaporator), and then, under the action of mechanical work (in the compressor) is compressed and raised to a sufficiently high temperature to permit the dissipation of this heat to the surrounding ambient (in the condenser). Accordingly, the system uses the refrigerant as a heat pump fluid that absorbs heat from a space or medium that is to be cooled, and dumps the recovered heat in another location.
The absorption of heat, and the cooling effect of the system, is produced in the evaporator as the vaporizing refrigerant absorbs heat, thereby cooling its surroundings. The evaporator may cool in a direct method, where the refrigerant acts in direct heat exchange to cool a space or a product; or it may cool by an indirect method, where the refrigerant is in heat exchange with a secondary medium such as water or brine, which is cooled by the refrigerant and then pumped to a more distant point to absorb heat.
The evaporator heat exchange structure may comprise a fin and tube construction for cooling air, or it may comprise a shell and tube construction for cooling a liquid. The shell and tube structure includes a set of tubes surrounded by a shell. The boiling (evaporating) refrigerant is carried inside the tubes, and the liquid to be cooled surrounds the tubes within the shell. The evaporating refrigerant is thereby circulated in heat exchange relation with the liquid within the surrounding shell. Because the evaporation of the liquid refrigerant is an endothermic reaction, the evaporating refrigerant will absorb heat from its surroundings. In the fin and tube evaporator structure the refrigerant removes heat from the air. In the shell and tube evaporator structure the evaporating refrigerant removes heat from the liquid to be chilled.
In the condenser, heat must be removed from the hot refrigerant vapor discharged into the condenser from the compressor, and the vapor must be condensed to a liquid. Because the condensation of a gas is an exothermic reaction, the condensing vapor will give off heat to its surroundings. As a result, the condenser dissipates heat from the refrigerant to a surrounding coolant, either to the ambient atmosphere (using a fin and tube structure) or to a circulating liquid (using a shell and tube structure). The temperature of the refrigerant vapor in the condenser is kept above that of the coolant by compression to ensure that heat is transferred to the coolant.
The expansion valve feeds the evaporator with a controlled flow of liquid refrigerant from the condenser. The controlled flow must allow a sufficient amount of refrigerant into the evaporator for the cooling load, but not in such excess that unevaporated liquid refrigerant passes into the compressor (which would damage the compressor). The flow rate of refrigerant from the condenser to the evaporator through the expansion device may be modulated by a temperature-controlled valve located between the condenser and the evaporator, near the evaporator inlet. Such a device is commonly known as a thermal expansion valve.
The thermal expansion valve is a diaphragm-operated valve with opposing pressures above and below the diaphragm causing the diaphragm to open and close an attached valve stem and seat. A pressure dome and cylinder within the valve defines a chamber above and a chamber below the diaphragm.
The pressure above the diaphragm is related to the temperature of the refrigerant leaving the evaporator. Above the diaphragm is a dome connected to a temperature sensing bulb through a capillary tube. The bulb, dome and capillary tube are filled with a refrigerant vapor and/or liquid that has similar pressure/temperature characteristics as the refrigerant in the system involved. The temperature sensing bulb is located at the outlet of the evaporator. Below the diaphragm is a connection to the evaporator inlet so that the pressure on the underside of the diaphragm is the pressure in the evaporator. A spring under the diaphragm or valve stem causes a bias towards the valve closed position.
As the temperature of the refrigerant vapor leaving the evaporator increases, the temperature of the expansion valve bulb increases. The increasing temperature of the bulb expands the fluid in the capillary tube. This increases the pressure above the diaphragm, urging the diaphragm against the bias spring and causing the expansion valve to move towards the open position, thereby passing more liquid refrigerant into the evaporator. The high temperature at the evaporator outlet ensures that vaporized refrigerant is being drawn into the compressor while liquid/vaporized refrigerant is flowing through the thermal expansion valve and into the evaporator at a controlled (relatively open) flow rate.
Conversely, as the temperature of the refrigerant at the evaporator outlet decreases, the temperature of the expansion valve bulb decreases. The decreasing temperature of the bulb decreases the pressure on the top of the diaphragm and allows the bias spring to close the valve, thus restricting the refrigerant flow into the evaporator. The decreasing temperature at the evaporator outlet warns that the possibility of passing liquid refrigerant to the compressor is increasing. The lessened refrigerant flow into the evaporator allows the evaporator to continue to vaporize all of the refrigerant and ensures that only vaporized refrigerant is being drawn into the compressor because of a controlled (relatively restricted) flow rate through the thermal expansion valve and into the evaporator.
Given the foregoing system components and process requirements, the pressure in the evaporator is determined by the process temperature which is to be maintained. The pressure in the condenser is determined by the temperature of the available cooling medium (circulating water or ambient air temperature). The refrigerant is a gas having a high critical temperature. Among the refrigerants in common use in systems such as the one described are the halogenated hydrocarbons: refrigerant-12 is dichlorodifluoromethane (CCl.sub.2 F.sub.2), known under the brand name, FREON-12. Refrigerant-22 is monochlorodifluoromethane (CClHF.sub.2). Refrigerant-22 operates at a higher pressure than FREON-12 and, in general, is used in higher temperature applications.
The foregoing discussion of a typical refrigerant vapor compression cycle included a system having a compressor, a condenser, an expansion valve, and an evaporator. A receiver is often added, and may be located after the condenser and before the expansion device.
The receiver is a storage tank, having an approximate volume capacity corresponding to that of both the evaporator and the condenser. The receiver acts as a reservoir for the refrigerant. During periods of low condenser ambient temperature, it is necessary to flood the condenser with liquid to reduce its capacity and maintain a desirable condensing pressure/temperature. Therefore, a reservoir must be provided in the system to accommodate the excess refrigerant during normal ambient temperatures.
Accordingly, a more complete refrigeration system for chilling a liquid may include a compressor, condenser, receiver, thermal expansion valve, and an evaporator. Although such systems are well known in the art, present systems do not generally start up quickly under low ambient temperature conditions, do not generally respond well under varying load conditions, and do not generally provide for effective use of the recovered heat.
In many applications, the difficulties of rapid start up under low ambient and steady performance under varying load are of relatively little concern. But, when the cooled liquid is used for precision cooling in such temperature-critical applications as electronic component production, photochemicals, plating, plastics, high speed drilling, machine tools, reclamation of volatile degreasing solvents, laser and x-ray cooling, these difficulties become a greater concern.
One way of addressing these concerns is by way of bypass valves, preferably hermetically sealed within the receiver, to selectively direct hot gaseous or liquid refrigerant to selected points within the system, dependent upon the temperature/pressure conditions being experienced. Examples of a receiver with bypass valves are shown or described in U.S. Pat. Nos. 4,689,969; 4,718,245; and 4,815,298, all under common ownership with this patent application. While each of those previous patents addresses the general problem, there is still room for improvement in the context of a liquid chiller.
It would be highly desirable, therefore, to provide a liquid chiller having the features of rapid system start up in low ambient temperature; steady operation under varying load conditions; and effective use of the recovered heat. It is a specific object of this invention to provide such features. These, and other, advantages of this invention will become apparent in the remainder of this disclosure.